Automatic test equipment for checking printed circuit boards has long involved use of a "bed of nails" test fixture in which the circuit board is mounted during testing. This test fixture includes a large number of nail-like spring-loaded test probes arranged to make electrical contact under spring pressure with designated test points on the circuit board under test, also referred to as the unit under test or "UUT." Any particular circuit laid out on a printed circuit board is likely to be different from other circuits, and consequently, the bed of nails arrangement for contacting test points in the board must be customized for that particular circuit board. When the circuit to be tested is designed, a pattern of test points to be used in checking it is selected, and a corresponding array of test probes is configured in the test fixture. This typically involves drilling a pattern of holes in a probe plate to match the customized array of test probes and then mounting the test probes in the drilled holes on the probe plate. The circuit board is then mounted in the fixture superimposed on the array of test probes. During testing, the spring-loaded probes are brought into spring-pressure contact with the test points on the circuit board under test. Electrical test signals are then transferred from the board to the test probes and then to the exterior of the fixture for communication with a high speed electronic test analyzer which detects continuity or lack of continuity between various test points in the circuits on the board.
Various approaches have been used in the past for bringing the test probes and the circuit board under test into pressure contact for testing. One class of these fixtures is a "wired test fixture" in which the test probes are individually wired to separate interface contacts for use in transmitting test signals from the probes to the external electronically controlled test analyzer. These wired test fixtures are often referred to as "vacuum test fixtures" since a vacuum is applied to the interior of the test fixture housing during testing to compress the circuit board into contact with the test probes. Customized wired test fixtures of similar construction also can be made by using mechanical means other than vacuum to apply the spring force necessary for compressing the board into contact with the probes during testing.
The wire-wrapping or other connection of test probes, interface pins and transfer pins for use in a wired test fixture can be time intensive. However, customized wired test fixtures are particularly useful in testing circuit boards with complex arrangements of test points and low-volume production boards where larger and more complex and expensive electronic test analyzers are not practical.
As mentioned previously, the customized wired test fixtures are one class of fixtures for transmitting signals from the fixture to the external circuit tester. A further class of test fixtures is the so called "grid-type fixture" in which the test points on the board are contacted by translator pins which contact random patterns of test points on the board and transfer test signals to interface pins arranged in a grid pattern in a receiver. In these grid-type testers, fixturing is generally less complex and simpler than in the customized wired test fixtures; but with a grid system, the grid interfaces and test electronics are substantially more complex and costly. It is the grid-type testers to which the present invention is directed.
A typical grid fixture contains test electronics with a huge number of switches connecting test probes in a grid base to corresponding test circuits in the electronic test analyzer. In one embodiment of a grid tester as many as 40,000 switches are used. When testing a bare board on such a tester, a translator fixture supports translator pins that communicate from a grid pattern of test probes in a grid base to an off-grid pattern of test points on the board under test. In one prior art grid fixture so-called "tilt pins" are used as the translator pins. The tilt pins are mounted in corresponding pre-drilled holes in translator plates which are part of the translator fixture. The tilt pins can tilt in various orientations to translate separate test signals from the off-grid random pattern of test points on the board to the grid pattern of test probes in the grid base. This grid pattern of test probes in the grid base communicates with the test electronics through the respective switches previously mentioned. In a typical grid-type test fixture the grid base may have its test probes arranged on 100 mil centers. The tilt pins used in the translator module make contact with a preselected number of test points on the board and then make contact with selected test probes in grid base, and as a result, a large number of switches which are connected to all of the probes in the grid base go unused during testing, which is a typical characteristic of grid-type test fixtures. Because the spacing density among test probes in the grid base is limited, test points on a board having a density greater than the typical 100 mil on-center grid base density, for example, cannot be easily tested on the standard 100 mil center grid-type fixture. For bare boards today which have closer and closer test point densities, it becomes extremely more difficult to test these boards on the standard grid-type test fixture.
One embodiment of the present invention provides a fixturing system for selectively modifying the test point density of a grid-type test fixture to provide accurate testing for greater test point densities than would otherwise be available with a standard translator fixture and the standard grid pattern. The invention includes an adapter that cooperates with the translator fixture to expand test point densities for boards or portions of boards having test points arranged in high density patterns, substantially higher than the density of the probes in the standard grid base. The adapter allows use of tilt pins to translate test signals from high density test points on the board under test to the lower density grid pattern of test points in the standard grid base.